That comes from NetBSD's PCI configuration code, I get it too with an
ex(4) in the O2's PCI slot. Doesn't seem to have any adverse effect
though, the ex seems to work although I didn't exactly torture it a lot.

Yeah, the ex(4) worked quite nicely; I thought I'd give the wm(4) a try
again since it's been ages since I did last time. I can always go back
to the ex(4) or rtk(4), which both were stable even as of 1.6-ish, but I
thought I'd give this one more whack.

I did see where that message comes from but don't yet have any clue as
to why -- was wondering if there was some bad PCI accounting that may be
affecting the wm(4).